In the case of loading trucks or ships with coated automobiles, the parts thereof, etc., and transporting them to remote places such as overseas, etc., an effective means for preventing the paint films from being damaged, dulled, discolored, etc., by various suspensions or collisions of dusts, rains, stone particles, etc., has been desired.
Hitherto, materials coated with a wax series coating materials at a thickness of from 5 to 20 .mu.m have been known as such a preventing means. However, this method has various problems that it is difficult to form the wax coating at a uniform thickness, whereby a uniform coated film is difficult to obtain, the coated film is liable to be stained and has a poor resistance to acidic rain, the application of the wax coating and the removal thereof require much labors, the use of a solvent, the treatment of waste liquids, etc., are liable to cause environmental problems, etc.
On the other hand, various surface protective sheets each comprising a supporting substrate having formed thereon a pressure-sensitive adhesive layer are known. A surface protective sheet comprising a supporting substrate having formed thereon a radiation-curable pressure-sensitive adhesive layer having a low glass transition point is proposed as a surface protective sheet for an adherend having a paint film as described in JP-A-2-199184 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Also, the present inventors proposed a paint film-protective sheet using a rubber-based polymer such as polyisobutylene as a pressure-sensitive adhesive as described in JP-A-6-73352 corresponding to U.S. patent application Ser. No. 08/368,401 filed Jan. 3, 1995. The problems described above can be overcome by such sheet systems.
However, there is a problem that according to the kind of the paint film, the paint film causes a problem of poor adhesion such that the paint film is poor in the adhesive force from the beginning or lowers the adhesive force with the passage of time to cause a peeling phenomenon, whereby, as the case may be, the paint film cannot substantially be practically used for the surface protection.
In the course of various investigations for overcoming the above-described problems, the present inventors have clarified that the above-described poor adhesion problem is caused by bleeding unreacted low molecular weight components in the paint film and the leveling agent compounded in the coating composition for preventing the occurrence of cissing at coating, on the surface of the paint film. That is, by such bleeding, a layer having a weak cohesive force is formed on the surface of the paint film and the layer inhibits development of the adhesive force.
If the above problem is overcome by a method of cleaning the surface of the paint film to remove the inhibiting materials and then adhering a surface protecting sheet to the surface of the paint film, much labors are required for the cleaning treatment, whereby the technical merit for the method of applying the wax paint film is greatly reduced. Thus, the inventors attempted to overcome the above problem by increasing the adhesive force. However, since it was difficult to balance the initial adhesive force and the easily peeling property at peeling, a satisfactory result could not be obtained.